The invention relates to a method of and to apparatus for shaping a fabric or plastic sheet material in a suitable mold under differential pressure so as integrally to incorporate the shaped sheet as the cover or facing of a foamed plastic cushion, protective padding element or similar article. The sheet material may be plastic, fabric, or combination thereof, where in any event the material is naturally, or is made to be, non-pervious to passage of air through it so that it may be shaped in a suitable mold by differential pressure applied across opposite faces. The mold consists of cooperating molding dies of appropriate cavity configuration such that when a cover or facing sheet is supported in and shaped to conform to its respective molding die, two or more of these dies can be brought together to define a mold cavity completely lined by the cover sheets and forming an enclosure shaped to the desired configuration of the finished cushion, vehicle seat, dashboard padding, etc. The fabric or plastic sheet lined mold cavity is then injected with a self-foaming liquid plastic mix which expands in the enclosure to fill the same and provide the internal support for the finished product with its facing sheet integrally attached.
In the production of cushions as for example those for chairs, vehicle seats or the like, a method has been proposed (see German patent application No. S 32 875 VII 56a) in which a conventional foamed plastic material or rubber is filled into a casing made of rubber or non-hardening (thermoplastic) material to form a moss-like cushion composition in the casing. The casing or cover is placed in a suitable mold configured to the desired shape of the finished article, and the mold is charged with elastomeric filler material while the casing is supported in the mold, whereby to keep the outline of the cushion. The casing itself consists of two or more three-dimensionally preshaped pieces which in their original condition are simply cloth-like thin sheets. A disadvantage of this arrangement lies in the fact that a separate operation and apparatus are required to provide the preliminary shaping of the sheets into their desired configured form, something which represents additional labor requirements and increased complexity of production equipment. A further disadvantage of this prior production method resides in the fact that after gelling of the elastomeric material, it is not possible immediately to remove the finished cushion from the mold since subsequent processing is required, particularly as regards connection of the edges of the sheet members and subsequent trimming or finishing operations in respect to the junction at the part line of the mold.
In another method heretofore proposed involving a composite surfacing structure of fabric or the like, in which a thermally deformable textile sheet material made impermeable to air by an elastic coating on one of its surfaces, is laid across the open face of a molding die and the sheet material is then brought into snug engagement by differential pressure to conform with the configured mold wall, thereby forming a lining for that wall. A self-foaming elastomeric mix is then poured into the lined mold cavity and allowed to develop in the cavity. When the polymerization of the elastomeric material is completed, the resulting cushion is removed from the mold with the fabric or other facing sheet permanently attached to the foam.
Although this method of production avoids the disadvantage inherent in the previously mentioned method of separately preshaping the cover sheet, there is still a difficulty in that the production of even, uniform extension or stretch in the facing material of the finished cushion presents practical problems. That is, when the facing sheet is initially subjected to pressure differential to preshape it to the mold cavity, the whole area of the facing sheet is more or less evenly stretched; but only those regions corresponding to the shallower regions of the mold cavity are brought into actual contact with the surface of the mold in this initial stage. However when the fabric contacts the shallower regions of the mold cavity, those regions of the cover sheet become locked by friction against further extension in the mold, even though adjacent regions then undergo further extension or stretching in order to conform them to the deeper regions of the mold cavity. This leads as a result to different degrees of extension in different regions of the fabric and therefore to locally different thicknesses of the facing material on the finished cushion. In a finished product such as a cushion, therefore, it is just those parts which are subject to heaviest loading and wear which are covered by the thinnest regions of the facing sheet.
It has been proposed heretofore to overcome this difficulty and obtain an even thickness of the facing sheet throughout its surface by allowing the sheet to slide on the mold wall during the preshaping operation. However this is not readily possible, particularly in cases where the mold is highly configured to provide substantially different levels in depth of the cavity. Nor is it practical to incorporate liquid or powdered lubrication between the cover sheet and the mold wall to allow slipping to occur more easily, as this causes fouling of the exposed surface of the finished cushion. It may also produce clogging of perforations in the mold wall required to transmit differential pressure to the undersurface of the facing material.
In order to deal with or at least reduce the abovementioned difficulties, it has heretofore been suggested to use a mold plunger, or a number of such plungers, which preliminarily depress the fabric toward the lower or deeper levels of the mold cavity before application of differential pressure to the cover fabric or facing sheet. By the use of such mold plungers the fabric is subjected initially to a more-or-less even deformation throughout its area while giving it a preliminary shape configured more nearly to the ultimately desired shape. As a result, when the cover fabric is brought into final contact with the total mold surface by differential pressure, there is only a comparatively small further deformation then required, so that undue stretching of the cover fabric is largely eliminated. As will be readily apparent however in the foregoing procedure, additional equipment for premolding or shaping of the cover fabric is required and this is relatively complicated and expensive. Furthermore, use of such preforming plunger equipment does not lend itself to automated, continuous molding operations employed in a production line consisting of a plurality of identical molding dies continuously traveled about an endless conveyor which advances the dies through successive processing stations and operations.
It has also previously been proposed (see German patent application No. P 25 11 811.0) to employ an elastically deformable perforated membrane of varying thickness between the fabric and the mold wall, wherein regions of the membrane corresponding to the deeper levels of the mold contour are thicker than those of the membrane corresponding to shallower levels of the mold contour. Owing to this thickness variation, the membrane has a correspondingly variable stretch capacity in its different regions, and this produces a relatively even stretch or extension of the cover sheet while the latter is frictionally held in engagement with the surface of the membrane so as to prevent lateral movement between them during the shaping and molding operations. A disadvantage with that method, however, is that it does not provide means for producing a totally covered cushion; that is, a cushion having both a front and a rear facing sheet, in one molding operation.